The name Petrianis comes from the cohort that was stationed there, which appears to be a latinisation of a Celtic toponym thought to mean High Fort.

The fort was later considerably expanded to the north and west to accommodate the larger Ala Gallorum Petriana,[5] the nominally 1,000-strong cavalry regiment, the sole cavalry regiment of this size in Britain, after the internal buildings had been demolished as well as the south-western and northern rampart walls.

The later fort layout extended north beyond the Wall to allow the garrison to exit northwards through 3 wide gates.

[9] Because of the large size of the fort, it is thought to have housed a cavalry regiment, nominally one thousand strong.

Traces of a civil settlement outside the walls are surprisingly slight given the large size of the fort, but the Roman town of Carlisle was probably near enough to serve in place of a vicus.

[12] Archaeological evidence includes masonry and pottery in the churchyard and material on Stanwix bank, second-century occupation found in 1931 outside the west rampart, and a mid 2nd-century bronzesmith's workshop discovered in 1930 below the fort with both military and civilian products.

[13][14] Excavations in 1932–4 traced the ditch for the south rampart as well as Hadrian's Wall, which formed the north face of the fort.

Also found was an inscription to Julia Domna, the wife of the Emperor Septimius Severus, whom she accompanied in Britain from 208 until his death in 211 at York.

[17][18] Later finds included a group of about 30 semi-precious gems, some carved as intaglia, which were found in a drain and are thought to have been lost by bathers.